Approximately 25-30% of U.S. veterans who served in the 1990?1991 Gulf War are affected by Gulf War Illness (GWI). GWI is a devastating and debilitating condition, where GWI symptoms present as both central nervous system (CNS) and peripheral symptoms, including widespread pain, musculoskeletal pain, headache, persistent cognitive deficits, fatigue, stomach and intestinal symptoms, respiratory complaints, and skin abnormalities. Recent studies support abnormal hippocampal pathology in GWI veterans with cognitive deficits, highlighting an important CNS link. One critical and prominent feature of GWI is that symptoms persist and in some cases progress, long after the Gulf War, now over 25 years later. Epidemiology supports that pesticide exposures, such as chlorpyrifos (CPF), may be linked to cognitive effects of GWI, but the underlying mechanisms are unknown. The neuroinflammation hypothesis of GWI cognitive deficits holds that CNS immune perturbation may drive the persistent CNS effects in GWI. Microglia, the resident myeloid (immune origin) cells in the brain, have been implicated as a chronic source of pro-inflammatory factors driving progressive neuron damage in diverse CNS conditions, including GWI. Recent reports indicate that peripheral injury augments the brain's pro-inflammatory response to cause neuronal pathology through circulating factors. Our over-arching hypothesis is that peripheral and CNS injury interact in GWI and in response to the GWI- relevant pesticide, CPF, where circulating damage associated molecular patterns regulate the chronic microglial pro-inflammatory response and consequent neuronal/memory deficits. Preliminary data indicate the chronic CPF GWI mouse model causes persistent neuroinflammation, chronic hippocampal synaptophysin loss, and neurobehavioral deficits 3 months after the last pesticide injection, validating the GWI model. Data also point to a role for HMGB1 in CPF effects, as the CPF GWI mouse model exhibited elevated circulating HMGB1 in bioactive serum, HMGB1 injected by tail vein elicited neuroinflammation/microglial activation, and HMGB1 directly triggered microglial activation in vitro. Mechanistically, microglia depletion was confirmed as neuroprotective against CPF in vitro. Thus, our specific hypothesis is that CPF exposure causes bioactive circulating factors (HMGB1), which then cause persistent microglial activation/neuroinflammation and GWI-like neuropathology. As such, the following AIMS will: 1) Define the neuroprotective efficacy of HMGB1 inhibition in a CPF model of delayed and persistent GWI-like neuropathology; 2) Confirm the role of myeloid cells in GWI- like neuropathology; 3) AIM3: Examine the neuroimmune bioactivity of GWI-like serum. These findings will provide much needed insight into the role of the periphery in the persistent response to GWI-relevant pesticides (CPF), implicate HMGB1 and temporary myeloid cell depletion as novel therapeutic targets, and begin to outline a neuroimmune bioactivity assay as a potential marker for GWI.